This invention relates to an ignition timing control apparatus for an internal combustion engine, and in particular to an ignition timing control apparatus for an internal combustion engine which is operable either with a low-octane rating fuel or a high-octane rating fuel.
It is well known in the art that the octane rating of fuel gasoline is greatly interrelated with a knock-resistibility in an internal combustion engine. Namely, as the octane rating of gasoline becomes higher, the knocking of an engine correspondingly decreases.
FIG. 1 shows an ignition timing vs output shaft torque characteristic in an internal combustion engine in the case where a commercially available low-octane gasoline (regular gasoline) and a commercially available high octane gasoline (premium gasoline) of which the octane rating is higher than that of the low-octane gasoline. In the figure, point A indicates a knock limit for the low-octane gasoline while point B indicates a knock limit for the high-octane gasoline, in which knock arises when the angle of the ignition timing is advanced beyond these points. According to FIG. 1, it is possible to advance the angle of the ignition timing up to a point B if the high-octane gasoline is used, where the output shaft torque will be higher as compared with the use of the low-octane gasoline.
FIG. 2 shows an ignition timing characteristic representing points A and B as a function of engine speed. It is seen that the interval between the characteristics A and B is constant as a function of the engine speed. Therefore, in an internal combustion engine with such a characteristic, it will be possible to increase the output of the engine by advancing the ignition timing by a predetermined angle if fuel used in the engine is changed over from the low-octane rating gasoline to the high-octane rating gasoline.
Also in the case where a mixed fuel of both gasolines is used for the engine, it will be possible to increase the output of the engine if the ignition timing is advanced in angle according to the mixture ratio of the fuel.
However, in a conventional ignition timing control apparatus for an internal combustion engine, the reference ignition timing characteristic is preset only for a predetermined gasoline, e.g. the low-octane rating gasoline. Therefore when using the high-octane rating gasoline, an increase in the output of the engine can not be expected without any change or modification thereof, so that the presetting of the reference ignition timing must be repeated towards the advance angle by some method.
Particularly, such a repeated presetting of the reference ignition timing is very hard when using the mixed fuel because, as shown in FIG. 2 by a dotted curve C, the knock limit exists between curves A and B according to the mixture ratio and so the possible advance limit is varied.
Even if the reference ignition timing has been again preset at the knock limits A and B, since the knock limits A and B in FIG. 1 may vary with environmental conditions during the operation of the engine such as temperature, humidity, etc, knocks easily arise at the time of transient operation such as an accelerated operation of the engine, and accordingly it is quite difficult to avoid the occurrence of engine knocks.
U.S. Pat. No. 3,822,583, titled "Method for Determining Octane Ratings of Fuels under Road Conditions" issued on July 9, 1974 to B. D. Keller et al. discloses an ignition timing control according to the octane rating of gasoline by a knock detection.